In cold climates particularly where occupied spaces must be heated, air in these spaces tends to have low relative humidity. This is uncomfortable and sometimes even unhealthy. To remedy this problem, people use humidifiers to add humidity to the air in these spaces.
Humidifiers have a variety of different designs. There are small stand-alone units intended for a single room. Larger units are designed for permanent installation as a component of a central heating system. These add moisture to the stream of heated air passing through the furnace duct to the occupied space. The latter type of humidifier will be referred to as an "in-duct" humidifier hereafter. The humidifier whose description follows is an improvement to one common type of in-duct humidifier.
There are a number of different designs for in-duct humidifiers. The kind which is involved here has an air-permeable pad, typically made from a number of similarly sized layers of thin expanded aluminum sheet stacked to a thickness of perhaps 1.5 in. (3.8 cm.). The layers of aluminum sheet are bonded to each other so as to create a pad structure having a rectangular box-like shape. Such a pad of expanded aluminum construction is rigid or semi-rigid, but relatively light and quite easily crushed. The pad is mounted in a housing which is placed in or near the furnace duct so that air warmed by the furnace can flow through the housing and the pad within it. Water is allowed to drip onto the top surface of the pad at a rate which keeps the pad moist from top to bottom. The warm air passing through the pad evaporates the water in the pad, adding humidity to the air. In some designs, the pad is first mounted in a frame which is then inserted into the humidifier housing.
The water drips onto the pad from what is called a water distribution tray, or simply a tray. The tray extends along the top surface of the pad and has a reservoir for holding a small amount of water. Water is fed to the tray from the building water supply, and flow is controlled by a solenoid valve. The tray has holes spaced along its bottom through which water flowing into the tray falls onto the top of the pad. By properly selecting the rate at which water is added to the tray, the pad can be kept moist from top to bottom. The pad, the tray, and a housing supporting them in the proper spatial relationship comprise the most important elements of an in-duct humidifier. It is important for efficient operation of the humidifier that the tray evenly distribute water across the entire width of the pad.
The minerals dissolved in the water which is fed to the tray and which wets the pad sometimes create a problem for humidifiers of many types including these pad-type humidifiers. We find that water seeping or spraying from the pad will dry on the interior of the housing and on the ductwork, eventually building up to a substantial thickness which may interfere with its operation. The use of a pad frame such as is shown in U.S. Pat. No. 5,211,891 issued to Anoszko on May 18, 1993 to isolate the pad from the housing is helpful in reducing the amount of such mineral buildup. But it is possible to limit mineral deposits even more effectively than is shown in Anoszko.